WASHINGTON — The U.S. Postal Service says it will delay plans to cut Saturday mail delivery because Congress isn’t allowing the change. The Postal Service said in February that it planned to cut back in August to five-day-a-week deliveries for everything except packages, as a way to hold down losses. But a statement today from the agency’s Board of Governors notes that Congress has passed a spending bill that continues the longtime prohibition against reducing delivery days. As a result, the board says it believes that Congress “has left it with no choice but to delay implementation” of the five-day-a-week plan.